A common problem associated with rare gas halide lasers is a decrease with time of the output energy due to the buildup of contaminant species within the laser discharge head. These contaminants either arise from reactions involving the halogen donor with materials used in the laser system or are introduced with the gas mixture of commerical grade gases. Removal of these impurities leads to dramatic increases in the gas life times and consequently improved laser performance and reduced running costs.
The impurities can be removed either by chemical gettering (a process which also removes the halogen donor and requires continued replenishment--Appl. Phys. Lett. 32(5) (1978) p 291-2) or by selective condensation. For this latter method it is fortuitous that the rare gases and the halogen donors used in rare gas halide lasers have significantly large vapour pressures at those temperatures at which most of the contaminant species can be condensed out of the gas mixture.
A trap operating at the fixed temperature of liquid nitrogen (77.degree. K.) has shown the efficacity of the technique for the cryogenic removal of contaminants in an ArF laser system (IEEE JQE Vol 16 p 231-4 (1980)). However at 77.degree. K. the vapour pressures of both xenon and krypton are too small to enable these component gases to take part in the rare gas halide laser activity, thus a trap operating at liquid nitrogen temperature cannot be generally employed in rare gas halide laser systems.
The condenser trap described for the argon fluorine (ArF) laser support system is one of simple construction, comprising a copper pipeline loop, part of which is immersed in a dewar of liquid nitrogen. The operational temperature of this trap is that of the liquid nitrogen refrigerant, and no regulation is afforded.